Overload control circuit



March 7, 1939. R KIRKWQOD 2,149,700

OVERLOAD CONTROL CIRCUIT Filed March 31, 1956 Es {Hillba rim E ao f I Patented Mar. 7, 1939 PATENT OFFlCE/j OVERLO'AD CONTROL CIRCUIT Loren R. Kirkwood, Colllngswood, N. J., assignor to Radio Corporation of Delaware of America, a corporation Application March 31, 1936, Serial No. 71,840

'1 Claims.

The present invention relates. to an overload control circuit for a radio receiver, and has for its' object to provide an improved circuit of that character which may operate automatically to prevent overload and distortion in the operation of amplifier tubes associated with the audio frequency detector.

The invention is particularly adapted for use in connection with radio receivers having a diode 1o rectifier type of second detector, the direct current potential from which is utilized to provide biasing potential for the succeeding audio frequency amplifier.- It is, therefore, a further object of the present invention to provide an improved biasing control circuit whereby an audio frequency amplifier following the second detector may be diode biased, while at the same time the amplifier tube may be provided with a fixed initial biasing potential in the absence of signals.

In the usual circuit of this character, the initial biasing potential is necessarily applied to and provides a delay on the second detector or diode rectifier, which delay is undesirable in certain cases. Accordingly, it is also an object of this invention to provide a signal diode bias circuit for a radio receiver or amplifier, in which an initial fixed bias potential, in the absence of signals, is prevented from being applied in any effective degree to the diode anode, thereby to cause any appreciable delayed rectifier action.

In a reflex type of radio receiver wherein an amplifier or other tube is used for the double function of radio or intermediate frequency amplifier and as an audio frequency amplifier, overloading of the tube, that is, causing the signal on the control grid to overcome the negative bias, may cause unstable operation resulting in a squeal or other undesirable sound from the reproducing device. It is, therefore, an object of the present invention to provide an improved bias potential control system for a reflex amplifier circuit, which effectively prevents unstable operation upon overload and permits the overload to be carried smoothly without excessive distortion.

It is also a further object of the present invention to provide meansv for deriving fixed and signal diode bias potentials for an amplifier tube which is provided with a common cathode for both the amplifier portion thereof and the diode rectifier portion thereof, in a conductive circuit connecting the grid, the diode anode and the fixed source of biasing potential.

The invention will, however,- be better understood from the following description when considered in connection with the accompanying drawing, and its scope will be pointed out in the appended claims. I

In the drawing, Figure 1 is a schematic circuit diagram of a portion of a radio receiving system, including the second detector, embodying the invention, and

Fig. 2 is a similar schematic circuit diagram of a portion of the circuit of Fig. 1 showing a modification thereof.

Referring to Fig. 1, 5 is a combined intermediate and audio frequency amplifier and diode rectifier second detector in one tube, which comprises a cathode 6, a pair of diode anodes I associated with the cathode, a control grid 8, a screen grid 9, a suppressor grid l and an output anode II. The tube is, therefore, of the so called double-diode, pentode type having a high amplification factor, and preferably is provided with a variable-mu control grid whereby it is adapted to operate as a high gain amplifier,

The device is connected through its control grid 8 with a tuned input circuit I2 for receiving signals or modulatedcarrier waves. In the present example, this is the tuned secondary circuit of an intermediate frequency amplifier output transformer l3, the primary circuit M of which may be connected in any suitable manner to the precedingintermediate frequency amplifier and other well known elements of a superheterodyne receiver, through input leads [5. The preceding elements of -the receiver, including the tuning apparatus, are not shown for the purpose of simplifying the drawing. I

The suppressor grid ill is connected with the cathode 6 which is, in turn, connected through a cathode lead l6 with the common return circuit of the receiving apparatus, which is indicated as the chassis ground I1. The negative anode supply circuit,-indicated by the terminal I8, is connected through a voltage drop producing resistor I9 with the common circuit return or ground as indicated at 20. The positive anode supply terminal 2| is connected through the positive anode potential supply lead 22 with the output anode I I through a voltage drop producing resistor 23 and the primary 24 of an intermediate frequency coupling transformer 25 and anode circuit lead 26. The screen grid 9 receives operating potential from the positive supply lead 22 through a series resistor 21 forming a filter in conjunction with a suitable by-pass capacitor 28 in connection with the grid lead 29, which also is extended to the remainder of the apparatus as indicated at 30. Likewise, the positive supply lead 22 is extended as indicated at 3|.

Intermediate frequency signals amplified in the device 5 are taken through the anode circuit 25 and are transferred back through the transformer 25 to the secondary 32 which is coupled through a coupling capacitor 33 with one of the diode anodes I, being the upper anode as shown in the drawing. The lower diode is connected to the cathode by the lead 34 and is therefore rendered ineffective. The upper diode is connected through a diode output resistor 35 with the cathode lead I6. It will be noted that the secondary 32 is connected at its low potential end with the cathode lead I6 through the ground return circuit 36. Therefore, the audio frequency or modulation signal component and the direct current component of the rectified signal or carrier wave appear across the diode output resistor 35 between cathode and a point of high potential at the anode end 31.

Both the direct current and modulation components are reapplied to the control grid 8 through a filter resistor 38 connected between the output terminal 31 and the low potential side or lead 39 of the input circuit I2 for the control grid 8, and afteragain being amplified at audio frequency, the signals are derived from the output circuit 26 across the impedance or resistor 23 which forms the output impedance for the audio or modulation frequency signals. In the present example, the modulation or audio frequency output impedance 23 is coupled through a coupling capacitor 40 with the grid circuit 4| of afollowing audio frequency amplifier 42. This may comprise another electric discharge amplifier device having its cathode 43 connectedto the common circuit return by the ground connection 44 and the output anode 45 may be coupled through an output transformer 36, with further amplifying means (not shown) which may be connected with the output leads 41.

The anode circuit for the amplifier device 42 is connected through a lead 48 with the positive supply lead 22, as is also the screen grid 49. The suppressor grid 59 is connected to the cathode 43 and the control grid 5| is connected with the input circuit ll. A suitable by-pass capacitor 52 is provided in connection with the coupling resistor 23 to by-pass any intermediate frequency component of the signal which may be present in the output circuit, whereby the same is prevented from reaching the control grid 5 I.

From the foregoing description, it will be seen that the circuit arrangement is such that the anode current from the various electric discharge or amplifier devices in the receiver is caused to fiow through the series supply resistor l9, thereby causing substantially fixed or constant potential drop therein and resulting in a diiference of potential between the common circuit return or ground 20 and the opposite end of the resistor at a potential supply point 53. The

polarity of the potential across the resistor I9 fixed value and the arrangement is such that the positive terminal is connected to the common circuit return or ground. Likewise, the signal variable potential supplied by the resistor 35 is positive at the ground or common circuit return side thereof and the terminal 31 is at a negative potential in the presence of signals. In the absence of signals, there is no potential drop through the resistor 35. The resulting polarity is as described because of the fact that the diode rectifier or second detector formed by the upper diode anode I and the cathode 8 is of the shunt output type, that is, the diode output resistor 35 is connected directly between the cathode and the diode anode. As hereinbefore described, the negative potential at the terminal 31 resulting from the reception of a carrier wave is applied to the control grid 8 through the lead 39 and the circuit I 2 from the filter resistor 38.

The diode rectifier may be of the series type as shown in Fig. 2, to which attention is directed, and in which the same circuit elements are provided.

The rectifier cathode 6 is connected to ground I I as in the circuit of Fig. 1 and the diode output resistor 35 is connected between the secondary 32 and ground, completing the diode circuit from the secondary directly to the upper anode The negative terminal 31 is connected with the filter resistor 38 as before and an intermediate frequency bypass capacitor 35a to ground is provided for the diode output resistor 35.

Referring to Fig. 1, it will therefore be seen that the audio frequency amplifier portion of the tube 5, is conductively connected to the diode output resistor and therefore is diode or signal biased from the second detector diode, by action of the potential existing across the diode output resistor 35.

In the absence of signals, however, it will be seen that the tube 5 would normally be without grid bias and would, therefore, tend to draw excessive plate current and, furthermore, in the presence of signals the signal strength might be such that the negative bias provided thereby would not be sumcient to prevent overloading the grid 8 at audio frequency. Accordingly, the source of biasing potential represented by the resistor I 9, is utilized to provide the initial bias potential and is in the proper polarity relation with respect to the common circuit return or ground to do this in cooperation withthe biasing potential later to be'derived from the received signals through the resistor 35.

The connection is readily made by connecting the filter output lead 39 through asupply lead 55 and a potential control resistor 56, to a potential supply terminal 58 to which the negative output terminal 53 of the fixed source I9 is also connected through a suitable filter comprising a series resistor GI! and by-pass capacitor 6|. With this arrangement, in the absence of signals, the negative potential existing at the terminal 53 may be applied to both control grids 8 and 5| through direct conductive circuit connection with the terminal 58. The connection for the grid 5| iscompleted through a grid resistor 51. It will be noted also that the filter resistor is provided with a bypass capacitor. 59 for intermediate frequency currents.

By proper relation of the resistance values provided by the resistors 56 and 38, the initial biasing potential available at the terminal 53 may be of desired value for properly biasing the grids 5| and 8, without providing an appreciable negative bias on the detector diode anode I, although the circuit connection which must necessarily be provided between the fixed and variable sources and the control grids, is conductive.

This circuit through the resistor network, from the fixed source l9, may be traced from the nega' tive terminal 53, through, the filteriresistor 80 to the terminal 58 which causes a relatively high negative potential to be applied to the control grid i through its grid resistor 51. For example, with an initial fixed potential of 20 volts negative at the terminal 53 and a filter resistor 60 of substantially .5 megohm and a coupling resistor 51 of the same-value, the initial negative voltage on the grid 5i may be of the order of 18 volts. Continuing the circuit, the series voltage drop producing resistor 56 is of such a value, for example, of the order of 5 to 6 megohms, that the potential applied to the control grid 8 from the lead 55 and the grid end of the filter resistor 38 may be of the order of 3.5 volts negative.

Tracing the bias supply circuit further through the filter resistor 38 and the diode output resistor 35 to the ground connection H, the same is returned to the ground 20 and the positive end of the source IS. The potential drop. in the filter resistor 38, which may be of the order of one megohm in resistance value, is such that the potential at the terminal 31 and at the upper diode anode I may be of the order of .5 to .7 volt negative, which is ineffectual to cause any degree of delay in the rectifier action of the diode. The output resistor 35 may be of the order of 220,000 ohms in the present example and the amplifier and detector device 5 may be of the type known on the commercial market as the RCA 6B7 tube.

The operation of the circuit shown, in the presence of a received carrier wave, is such that as the rectified carrier wave appears across the diode output resistor 35, the negative potential at the terminal 31 gradually rises in response thereto and reaches a value dependent upon the strength of the received carrier wave and in opposition to the biasing potential supplied by the fixed source 18, whereby the voltage drop from the latter source through the series resistor 56 and the filter resistor 38 is gradually reduced in proportion thereto and whereby the grid- 8 of the amplifier 5 receives an increasing higher negative potential in addition to the initial biasing potential received from the source I9. Likewise, the biasing potential applied to the grid 5| may'rise slightly because of the reduced drop of potential through the filter resistor 60.

From the foregoing description, it will be seen that the two sources of biasing potential are electrically separated in a conductive circuit through a series of voltage drop producing elements which operate to place a relatively low or substantially.

' more stable in that it prevents the control grid of the reflexed tube 5 from being overloaded by excessive signal voltage.

The resistors 38 and 58 together with the filter resistors, in efi'ect provide a bleeder resistor, the proportions of which determine the amount of fixed bias on the amplifier tubes and in the present example, in particular on the first audio frequency amplifier inthe device 5 which is diode signal biased, so that it will not overload and cause any undesirable sound characteristic with normal or higher than normal increase in signal input. Furthermore, the system provides means for obtaining fixed and diode bias on a tube which has a common cathode with the diode.

Because of the increasednegative bias supplied to the amplifier device 5 in the presence of signal, the same operates as automatic volume control means to reduce the gain in the tube for the intermediate frequency signal, as well as for audio frequency signals, whereby a desirable automatic volume control action is provided without the addition of suchdevice specifically in the receiving system. Accordingly, the circuit shown and described is particularly adapted to radio receivers which are of a simplified construction and of low cost, while providing a relatively high degree of eificient performance.

If desirable, additional automatic volume control potential may be derived for other circuits through a connection indicated by the lead 62 having a filter provided by the series resistor 83 and bypass capacitor 64.

It should be pointed out that the filter resistor 38 is preferably relatively low in resistance value with respect to the controlling resistor 56 in order that the potential on the control grid 8 of the diode biased amplifier may become substantially the same as or near to the value of the potential provided by the diode rectifier at the negative terminal 31 in a circuit such as shown in either Fig. 1 or Fig. 2. Furthermore, this resistor arrangement is desirable for the reason that in the absence of signals, the negative potential applied to the diode anode from the fixed source is rendered relatively low and substantially zero. Stated in another way, the bias control circuit is such that the fixed and variable sources 19 and 35 are connected in a resistance network which is tapped to provide initial and controlled potentials for the amplifier grids, and substantially no initial potential on the diode anode of the variable source.

The potential source I9 having substantially fixed value and the variable source 35 are of relatively low resistance with respect to the series circuit between them and operate in opposition to provide a fixed initial biasing potential on the amplifier grids. The drop in the series resistor 56 permits the negative bias potential to rise from the fixed initial value on the diode biased amplifier grid to a higher value directly upon reception of a modulated carrier wave. This has the advantage that the diode bias obtains control to increase the bias potential without first reducing the initial bias potential to zero. Overload of the diode biased amplifier and better audio frequency quality results in use of this system.

' I claim as my invention:

1. An overload control circuit for a diode biased electric discharge device, comprising means providing a source of negative potential of substantially fixed value, a diode signal rectifier, having an anode, a cathode and low resistance output impedance means providing a source of signal variable negative potential, voltage drop producing network interconnecting said sources of potential and providing a relatively high resistance with respect to the resistance of said output impedance, and an electric discharge device having a control grid connected with a point on said network having a relatively low resistance path between said point and the negative terminal 0! said diode variable potential source, and said diode anode being connected with a point on said network having substantially zero initial negative potential in the absence of signals.

2. In a radio receiving system, the combination with a diode rectifier and an electric discharge amplifier having a control grid, of means for applying a modulated signal wave to said diode rectifier, a relatively low resistance diode output impedance for said rectifier, a series relatively high resistance filter resistor connected between said output impedance and said amplifier for applying a signal variable biasing potential to the control grid of said amplifier, a source of fixed biasing potential connected with the amplifier end of said filter resistor, and a series controlling resistor in said connection having a resistance relatively high with respect to the resistance of said filter resistor and output impedance.

3. In a radio receiving system, the combination of an electric discharge device having a cathode, a diode anode associated therewith, a control grid and an output anode, means for applying a high frequency modulated signal to said control grid including an input circuit having a low potential lead, means for coupling said output anode electrode to the diode anode, an output resistor in circuit with said diode anode having a negative terminal providing a signal variable source of negative biasing potential, a relatively low resistance source of fixed biasing potential having a negative terminal, and a resistor network connecting said negative terminals and including a series of. resistance elements at least one of which is of relatively high resistance value with respect to the remainder of said elements, means providing a connection for said low. potential lead with said resistor network between said high resistance element and a second resistance element of said network more adjacent to the negative terminal of said variable source, and means providing a connection for the cathode with a positive potential point on each of said potential sources.

4. A receiving system in accordance with claim 3, further characterized by the fact that it includes a second amplifier device having a control grid, means connecting said control grid with a point, on said resistor network between said high resistance element and a resistor element more adjacent to the negative terminal of the fixed source of biasing potential, and means in circuit with said output anode electrode and said last namedgrid providing coupling therebetween for the transmission 01' the modulation component of the signal.

5. In a radio receiving system, a diode signal rectifier circuit having an output resistor providing a low resistance source of biasing potential responsive to variations in received signal strength, means providing a source of biasing potential of substantially fixed value, a filter re sistor of relatively higher resistance value with respect to the output resistor and a series control resistor of relatively higher resistance value connected in circuit between the negative terminals of said sources, said last named resistor having a resistance value such that it provides a major portion of the potential drop between said terminals, and an electric discharge signal amplifier having a control grid connected with a point between said last named two resistors and having a cathode connected with the positive terminals of said sources for amplifying and controlling the signals received thorugh said system.

6. In a radio receiving system in accordance with claim 5, further characterized by the fact that the resistances of said network between the said diode anode and the cathode, between the control grid and the diode anode, and between the negative terminal of the fixed source and the control grid are progressively higher with respect to each other in the order named.

7. In a radio receiving system, the combination of an audio frequency detector of the diode type for deriving the modulation component from a modulated signal wave, a diode output resistor therefor providing a direct current biasing potential in response to a rectified signal wave, said resistor having a grounded positive terminal, means providing a direct current biasing potential of substantially constant value havinga grounded positive terminal connection, means providing a relatively high resistance connection between the negative terminal of said lastnamed means and the negative terminal of the diode output resistor, an electric discharge amplifier device having'a grounded cathode and having a control grid connected to the negative terminal of the diode output resistor through a portion of said high resistance connection, said portion of the high resistance connection having a resistance relatively high with respect to the resistance of the diode output resistor.

LOREN R. KIRKWOOD. 

